In general, the pressure of gases and liquids is an important parameter to be controlled in numerous fields of application such as transport, energy, defense, health or computer-integrated manufacturing. This accounts for why numerous technologies of pressure sensors have been developed and why research work is ever important to improve the performance thereof.
So-called mass markets (automotive, electro-domestic, altimetry . . . ) above all demand price reductions. So-called professional markets (aeronautical, computer-integrated manufacturing, oil prospecting, research . . . ) demand ever more accuracy and resistance to severe environments, in particular to chemical attack by all fluids. All the fields request more miniaturization.
Ideally a pressure sensor must make it possible to obtain at one and the same time:                excellent accuracy;        good resistance to chemical attack of the environment;        good resistance to temperature;        excellent stability;        wide bandwidth;        minimum bulkiness;        low cost price.Measurement Principle        
In most cases, a pressure sensor incorporates a membrane which deforms under the action of a pressure that may typically be exerted by a fluid, this deformation being measured by virtue of resistive strain gauges deposited on the membrane. The gauges change resistive value while following the deformation of the membrane. Four gauges are generally used, mounted as a Wheatstone bridge and positioned in such a way that, under the effect of the deformation, two gauges increase in value and two others decrease. The power supply and the output of the bridge are connected by virtue of contact pins and/or conducting wires. These wires are attached to the membrane by virtue of connection pads which form the bond with the strain gauges.
The sensor generally comprises a so-called upper part called a cap exhibiting openings through which contact pickups can pass. Moreover the sensor can also incorporate a lower part called the “connection” intended for connecting it to the client's application. Advantageously this connection is equipped with a threading (or tapping) and exhibits an opening opposite a part of said membrane on which a pressure can be exerted.
The pressure difference between the two faces of the membrane is thus measured. In the particular case where the reference pressure applied to one of the faces is vacuum, the sensor is said to be absolute. For the so-called gage sensor, one of the faces of the sensitive element is referenced to atmospheric pressure.
There exist three large families of technology for manufacturing sensors with variation of resistors: so-called silicon membrane based on silicon technologies, so-called metallic membrane based on thin film technologies and finally, so-called ceramic membrane based on thick film technologies.
The latter exhibits a definite benefit with a view to cost optimizations but significant limitations in terms of fluidic compatibility, miniaturization and also resistance to high pressures and temperatures.
The main benefit of silicon technologies is that they utilize microelectronics miniaturization and cost reduction resources. Today they address the crux of the mass markets. The piezoresistive gauges are either diffused in the membrane (ionic implantation of N or P impurities), or made of monocrystalline silicon on insulator (SOI) for high-temperature applications. They give a higher output signal than thin film gauges but their value also varies more with temperature. Their final metrological performance is overall comparable to thin film sensors in terms of accuracy.
Thin film technology is differentiated in particular from the previous two in that its substrate is metallic. It is therefore naturally compatible with the very great majority of fluids used in industry. Whatever this fluid, it is in direct contact with the measuring membrane, equipped with the strain gauges, without the intermediary of a separator membrane such as that with which silicon sensors are equipped in the presence of corrosive fluids. Thin film technology sensors moreover exhibit the characteristics of high resistance and accuracy over wide temperature range. The signal that they deliver is weaker than the silicon sensors but exhibits the advantage of great stability over time.
State of the Art of so-Called “all-Fluid” Sensors
A significant limitation for silicon sensors comes from the very poor resistance of silicon to corrosive fluids. The manufacturers of pressure sensors who use silicon technology in harsh environments, circumvent this problem by protecting the silicon membrane in a stainless metal body. FIG. 1 thus illustrates a sensor of the known art exhibiting a silicon membrane.
More precisely this sensor comprises:                a connection 1;        a silicon membrane 2;        a stainless metal housing 3 enclosing said membrane forming cap in part;        intermediate elements 4 made of a material that may be glass and with expansion coefficient close to silicon, which are sealed in the housing by a flexible material 5;        pins 6 allowing contact pickups for the gauges;        an incompressible fluid 7 which transmits the pressure P to the silicon membrane;        a thin and flexible diaphragm 8;        wiring elements 9.        
Such a solution works, but however considerably reduces the expected miniaturization and cost reduction advantages of micro-technologies. Moreover, the flexible diaphragm and oil-filled design limit the accuracy, the frequency response and the temperature resistance of the sensor. Moreover, these intermediaries constitute weak points and may prohibit their use in specific applications, having regard to the risk of pollution of the process by the fluid 7 of the sensor or of instability and of inaccuracy if the sensor is subjected to fast thermal variations. Another difficulty for silicon sensors is related to the very large difference in expansion coefficient between silicon and metals. In many applications the sensor must be mounted on metallic walls by a thread connection. The silicon chip must therefore be fixed in a metallic housing, the difference in thermal expansion coefficient between the two materials then generating parasitic stresses and strains, sources of significant drift. This question forms the subject of a great deal of attention on the part of the manufacturers of silicon sensors who minimize the parasitic strain by interposing between the silicon sensor and the metal a sufficiently thick (1 to 2 mm) material (element 4 represented in FIG. 1), generally glass, with expansion coefficient close to silicon. This material is sealed in the housing by a flexible material 5 which thus absorbs part of the difference in expansion. The use of these intermediate materials can also lead to instability of the measurement over time.
The technologies of thin film sensor based on metallic membranes, stainless steel, titanium, hastelloy, inconel or copper-beryllium, are the oldest and exhibit the advantage of being directly usable with the majority of corrosive fluids. In their case, the measuring membrane equipped with the strain gauges is in direct contact with the fluids, without intermediary or protection, as is the case with the incompressible fluid and flexible diaphragm design for silicon sensors. Consequently, they are especially used for applications which require high resistance to chemical attack, good accuracy and reliability. Their general principle is recalled hereinafter and illustrated in FIG. 2.
The pressure membrane 2 is assembled with a thread connection 1. A stack of thin layers, one of which is an electrical insulator layer, the strain gauges 10 and connection pads 16 are deposited by (chemical or physical) vapor phase vacuum methods on this metallic membrane. The constituent materials of the gauges in particular can be made of thin layers of metallic alloys (deposition by cathodic sputtering of NiCr for example) or of semi-conductors (deposition of polycrystalline silicon for example).
The cap is produced with a metallic body 3 on which contact pins 6 are sealed by the glass-metal sealing technique, via glass sealing elements 15. The connecting of the gauges to the pins is performed by a wiring 9 produced by brazing of conducting wires. This assembly allows the creation of the reference pressure cavity 17: vacuum for absolute sensors or atmospheric pressure for gage pressure sensors.
A variant of the sensor with metallic membrane presented above is illustrated in FIG. 3. This variant makes it possible to circumvent a brazing (typically based on tin) performed directly on the sensitive element. A relay printed circuit 18 on which wiring can be produced by “ball bonding” 19 (welding of a wire or by ultrasound assisted thermo-compression) is used in this case. This system is preferable for the temporal stability of the layer but exhibits obvious drawbacks in terms of bulkiness.
The great benefit of these sensors is that they are metallic and, consequently, compatible with a majority of the aggressive fluids used in industry. However, a significant obstacle remains to be overcome for these technologies: very advanced miniaturization to obtain sensors of the order of 5 mm and even less in diameter.
State of the Art of Miniature Sensors
On the market, there exists mainly one type of pressure sensor with miniature metallic membrane 2, produced in accordance with a few variants. The metallic membrane is designed to be welded flush typically on a thread miniature metallic connection of type M5 or equivalent as illustrated in FIG. 4, which also depicts the gauge 10 and the insulator I located on said membrane 2.
The membrane is previously insulated and equipped with silicon bar gauges. For obvious space reasons, the Wheatstone bridge can be composed of two active gauges (“half bridge” setup), supplemented with offset fixed resistors.
This optimization of the setup makes it possible to address the miniaturization and fluidic compatibility requirements while offering a large dynamic range.
However, these models do not use thin film technology and therefore exhibit the drawbacks related to glued silicon bar gauges, very often crippling:                the pickup of connections is very complex and has an impact on reliability;        the sensor is sensitive to temperature variations and especially to thermal shocks;        the process for manufacturing the sensor is essentially manual;        the glue used to affix the silicon bar gauges to the metallic membrane induces creep over time and a limitation in temperature;        
In the absolute sensor case, the vacuum cavity cannot be produced at the level of the silicon gauges, as close as possible to them, thereby limiting the miniaturization possibilities.
Silicon membrane technology also proposes a few miniaturization solutions. FIG. 5 describes a typical state of the art in this field. The silicon chip consists of a silicon membrane 2 with deformation measurement gauges 10 made of doped monocrystalline silicon. A glass cap 11 is hermetically sealed by electrostatic bonding (“anodic bonding”) on the diffused silicon connection layer 12, thus protecting the gauges 10 from the exterior environment. Openings produced in this glass allow this assembly to be electrically linked to contact pins 6 by “glass frit” or “sintered glass” conductor 13 (mixture of gold and sintered glass). Such a solution enables extra miniaturization, however it still involves a stack of heterogeneous materials, with complex mounting operations. It does not on the other hand afford any progress as regards resistance to corrosive fluids.
It is in particular to achieve the dual-objective of miniaturization and “all-fluids” compatibility that the Applicant has designed a novel type of sensor with metallic membrane which, because of a compact architecture that can be miniaturized, makes it possible:                to ensure with few elements a pickup of exterior contacts of the measurement gauges;        to be assembled readily with any type of mechanical connection.        
In variants of the invention, the sensor is proposed wireless, thereby adding to the pluses related to its thin film technology very great robustness to stresses and strains from vibrations, accelerations or shocks.